1. Field of the Invention
This invention relates generally to magnetoresistive sensors for reading information signals from a magnetic medium and, in particular, to giant magnetoresistive and magnetic tunnel junction sensors with a Coxe2x80x94Fe/Supermalloy free layer.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of the SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1. This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant antiferromagnetic materials such as NiO for the AFM layer.
Referring to FIG. 2, an AP-pinned SV sensor 200 comprises a free layer 210 separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic layer 226 and a second ferromagnetic layer 222 separated by an antiparallel coupling (APC) layer 224 of nonmagnetic material (usually ruthenium (Ru)). The two ferromagnetic layers 226, 222 (FM1 and FM2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 227, 223 (arrows pointing out of and into the plane of the paper respectively).
Yet another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 3 shows a prior art MTJ sensor 300 comprising a first electrode 304, a second electrode 302, and a tunnel barrier 315. The first electrode 304 comprises a pinned layer (pinned ferromagnetic layer) 320, an antiferromagnetic (AFM) layer 330, and a seed layer 340. The magnetization of the pinned layer 320 is fixed through exchange coupling with the AFM layer 330. The second electrode 302 comprises a free layer (free ferromagnetic layer) 310 and a cap layer 305. The free layer 310 is separated from the pinned layer 320 by a non-magnetic, electrically insulating tunnel barrier layer 315. In the absence of an external magnetic field, the free layer 310 has its magnetization oriented in the direction shown by arrow 312, that is, generally perpendicular to the magnetization direction of the pinned layer 320 shown by arrow 322 (tail of the arrow that is pointing into the plane of the paper). A first lead 360 and a second lead 365 formed in contact with first electrode 304 and second electrode 302, respectively, provide electrical connections for the flow of sensing current Is from a current source 370 to the MTJ sensor 300. A signal detector 380, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 360 and 365 senses the change in resistance due to changes induced in the free layer 310 by the external magnetic field.
The use of Coxe2x80x94Fe (Co90xe2x80x94Fe10) for the free layer material in both GMR and MTJ sensors is very desirable since a higher GMR coefficient or magnetic tunneling coefficient is obtained with this material. However, the material Coxe2x80x94Fe has a high magnetocrystalline anisotropy constant, k, and therefore a high anisotropy field, Hk=2k/M, where M is the magnetization. When Coxe2x80x94Fe is used to fabricate the free layer of either GMR or MTJ sensors, the high Hk of the Coxe2x80x94Fe material results in a free layer having a high degree of magnetic stiffness. A magnetically stiff free layer is less sensitive for reading magnetic signal fields from the data magnetically stored on the disk since higher fields are needed to rotate the direction of the magnetic moment of the free layer in response to the signal field. To overcome this difficulty, prior art SV sensors have used a multilayer laminated free layer having a very thin sublayer formed of Coxe2x80x94Fe deposited adjacent to the spacer layer and a thicker sublayer formed of Nixe2x80x94Fe deposited on the Coxe2x80x94Fe layer. The thin Coxe2x80x94Fe sublayer of the laminated free layer improves the GMR coefficient while the thicker Nixe2x80x94Fe sublayer having a relatively low Hk results in an acceptable small net or effective Hk for the free layer of the sensor.
However, the need for higher data storage density requires GMR or MTJ sensors having very thin free layers. As the total free layer thickness is decreased, the thickness of the Coxe2x80x94Fe sublayer must be decreased to less than 10 xc3x85 in order to obtain the desired low stiffness free layer. Manufacturability of sensors having critical sublayers with thicknesses less than 10 xc3x85 becomes difficult because of the difficulty of controlling layer thickness to better than +/xe2x88x922 xc3x85 across the large diameter substrate wafers used in batch production of the sensors. Manufacturing yields of GMR and MTJ sensors are significantly reduced by variations of sensor characteristics caused by these layer thickness tolerance problems.
Therefore there is a need for GMR and MTJ sensors having layer thicknesses that are manufacturable at high yields while maintaining desirable free layer sensitivity and improved GMR and MTJ coefficients.
It is an object of the present invention to disclose an MTJ sensor having a laminated free layer comprising a first sublayer formed of Co90xe2x80x94Fe10 (Coxe2x80x94Fe) and a second sublayer formed of Ni79xe2x80x94Fe16xe2x80x94Mo5 (supermalloy).
It is another object of the present invention to disclose an MTJ sensor having a laminated free layer comprising a Coxe2x80x94Fe first sublayer and a supermalloy second sublayer having an increased magnetic tunneling resistance coefficient due to an increased Coxe2x80x94Fe sublayer thickness while maintaining a low effective Hk for the free layer.
It is a further object of the present invention to disclose an MTJ sensor having high sensitivity and increased magnetic tunneling resistance coefficient that is manufacturable at improved yields.
It is yet another object of the present invention to disclose an MTJ sensor having improved sensitivity due to a reduced stiffness of the free layer.
In accordance with the principles of the present invention, there is disclosed a magnetic tunnel junction sensor having a laminated multilayer free layer comprising a first sublayer formed of Coxe2x80x94Fe and a second sublayer formed of Nixe2x80x94Fexe2x80x94Mo. The Nixe2x80x94Fexe2x80x94Mo material of the second sublayer has a magnetocrystalline anisotropy constant, k, that is a factor of ten smaller than that of Nixe2x80x94Fe. Due to small value of k of the Nixe2x80x94Fexe2x80x94Mo material used to fabricate the second sublayer of the free layer, the thickness of the first sublayer made of Coxe2x80x94Fe may be increased to improve manufacturability without sacrificing low stiffness of the free layer needed for a high sensitivity of the MTJ sensor in response to signal fields from data magnetically recorded on a disk.
In the preferred embodiment of the present invention, the free layer of an MTJ sensor comprises a first sublayer formed of Co90xe2x80x94Fe10 having a thickness of about 10 xc3x85 deposited in contact with a tunnel layer layer and a second sublayer formed of Ni79xe2x80x94Fe16xe2x80x94Mo5 having a thickness of about 74 xc3x85 deposited on the first sublayer. The effective anisotropy field, Hk, of the laminated Coxe2x80x94Fe/Nixe2x80x94Fexe2x80x94Mo free layer having this structure is the same as the Hk of the free layer of prior art MTJ sensors having laminated Coxe2x80x94Fe/Nixe2x80x94Fe free layer structures in which the Coxe2x80x94Fe thickness is only 7 xc3x85. The 42% increase in thickness of the Coxe2x80x94Fe sublayer of the present invention substantially improves manufacturing yields of batch production of the SV sensor of the present invention.
Advantages of the MTJ sensor of the present invention employing a Coxe2x80x94Fe/Nixe2x80x94Fexe2x80x94Mo laminated free layer include improved manufacturability, a higher magnetic tunneling resistance coefficient, and low stiffness of the free layer in response to signal fields.
The above, as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.